The present invention relates to a process for manufacturing sprocket assemblies for power transmission chain systems. More particularly, the invention relates to a process for manufacturing multi-tiered and/or phased sprocket assemblies using capacitor discharge welding.
Power transmission chains are widely used in the automotive industry in automobile transmission systems as well as in engine timing drives. Engine timing systems conventionally include at least one driving sprocket located on the crankshaft and at least one driven sprocket located on a camshaft. Rotation of the crankshaft causes rotation of the camshaft through a chain and sprocket system.
Another type of engine timing system connects the crankshaft with two overhead camshafts by a chain and sprocket system. The crankshaft connects directly to the camshafts or through an idler sprocket. In an idler sprocket system, the idler sprocket and one sprocket of each camshaft are conventionally machined on the same spline or hub. Rotation of the idler sprocket therefore causes rotation of both of the camshaft sprockets. The idler sprocket is sized to allow different rotational speeds of the crankshaft and the camshafts.
The noise associated with chain drives may be generated by a variety of sources, including the impact sound generated by the collision of chain and sprocket at the onset of meshing, and the chordal action of the chains and sprockets. The loudness of the generated impact sound is affected by, for example, the impact velocity between chain and sprocket, and the mass of the chain links contacting the sprocket at a particular moment or time increment. The meshing impact sound is generally a periodic sound that is repeated with a frequency generally equal to the frequency of the chain meshing with the sprocket. The frequency is related to the number of teeth on the sprocket and the sprocket speed. The impact can therefore produce sound having objectionable pure sonic tones.
Chordal action occurs as the chain link enters the sprocket from the free chain. The meshing of the chain and sprocket at the chain mesh frequency can cause a movement of the free chain or span (the part of the chain between the sprockets) in a direction perpendicular to the chain travel but in the same plane as the chain and sprockets. This vibratory movement can also produce an objectionable pure sonic tone at the frequency of the chain mesh frequency or a derivative thereof.
Many efforts have been made to decrease the noise level and pitch frequency distribution in chain drives so as to minimize the objectionable effects of the pure sonic tones. For example, U.S. Pat. No. 5,427,580, which is incorporated herein by reference, discloses the phasing of sprockets so as to modify the impact generated noise spectrum as well as the chordal action noise spectrum. The assemblies made using the process of the present invention may utilize concepts taught in U.S. Pat. No. 5,427,580, in an idler sprocket system.
In a phased chain system where the chains are offset by a portion of a pitch length, there is phasing of the sprockets of two overhead camshafts with respect to one another along the idler shaft. Phasing the camshaft sprockets can reduce the number of chain link teeth (or mass of chain) impacting the sprockets at the idler shaft during a given time increment. Similarly, phasing the sprockets can alter or phase the chordal or articulation of the chains and sprockets, as well as the resulting impact and chordal action generated noise.
Prior art chain drives have provided for the phasing of the overhead camshafts. However, in these chain drives, the idler sprocket and one sprocket of each camshaft are machined on a single hub along the idler shaft. Such a system requires a complicated manufacturing process to machine three sprockets on a single hub.
The selection of a sprocket manufacturing process and associated costs are influenced by sprocket geometry. For example, the need for a middle chain groove in phased sprockets contributes to the cost differential between phased and conventional sprockets. Conventional sprocket manufacturing processes are inadequate to manufacture sprockets having complex geometries and integrated features such as, for example, conventional multi-tiered sprockets (e.g., idler, balance shaft) or phased sprockets. For example, in conventional stamping and fine blanking processes, the maximum part (e.g., tooth) thickness achieved is approximately 5.00 mm. Moreover, since the use of a GRIP-FLO stamping process is limited to sprockets having simple geometries with few integrated features, it cannot be used for many silent chain applications involving complex geometric configurations and integrated features.
Powder metal technology generally comprises the formation of metal powders which are compacted and then subjected to an elevated temperature so as to produce a sintered product. See, e.g., PCT WO 94/05822 to Shivanath et al.; PCT WO 94/14557 to Jones et al.; and PCT WO 95/14568 to Hinzmann et al.
However, powder metal is difficult to weld because of its porosity. Porosity typically results in residual gases, contaminants and lubricant becoming involved with the weld. Powder metal is also difficult to weld because of its high carbon content, which, for example, results in transfer of carbon to the weld and brittleness and cracks in the material upon cooling. Thus there is a need to develop an advanced powder metal process which can sustain increasing system loading and expanded durability requirements.
Further, the use of traditional powder metal compacting technology to form complex geometries like phased sprockets in one piece has been unsuccessful. One general limitation of traditional powder metal compacting technology is the lack of ability to position the powder in the forming tooling specifically and repeatably without cold working or pre-forming. Any initial working of the powder results in laminations or flaws in the final part structure that adversely affect the performance of that part. Thus, there exists a need for an accurate and efficient process for manufacturing sprocket systems having complex geometries.
The welding operation in a capacitor discharge welding machine is known to those of ordinary skill in the art. Typically, capacitor discharge welding or pulse welding is a form of resistance welding that is achieved within milliseconds at very high current levels by utilizing energy stored in a capacitor battery or bank. See, e.g., U.S. Pat. No. 3,641,305 to Ritter et al.; U.S. Pat. No. 3,838,786 to Bachmann et al.; U.S. Pat. No. 3,852,559 to Tauern; U.S. Pat. No. 3,984,653 to Blaas et al.; U.S. Pat. No. 4,132,879 to Glorioso; U.S. Pat. No. 4,672,164 to Devletian; and U.S. Pat. No. 5,359,771 to Krehl et al.
In general, during capacitor discharge welding, electrodes are conveyed towards the mating parts or work pieces to be joined, and an extremely short pulse is given. This very quick pulse of energy heats the surfaces of both components to be welded to a plastic state, thus avoiding the mixing and creation of undesired alloys at the weld interface. The weld is typically under high pressure (e.g., 40-60 tons for typical chain sprockets), which creates a high force, low inertia weld head that yields a joint similar to those found in diffusion welds. Typically, the work pieces display no distortion or dimensional changes after capacitor discharge welding. The acceptability of the weld may be determined by, for example, techniques described in U.S. Pat. No. 4,086,817 to Jon et al.
During capacitor discharge welding, a welding projection is required on one external surface in order to create small localized areas of high heat generation, thereby minimizing any negative effects from residual heat (e.g., hardness reduction, stress cracking). Martensite that forms from high carbon and high alloy welds can be instantaneously tempered with a second pulse, reducing the hardness and eliminating any source of weld failure associated with the high carbon weld. A corresponding welding depression on a second external surface may also be employed to receive the welding projection prior to welding; the welding projections and depressions facilitate alignment of sprocket assembly components, and accommodates excess material present at the weld site.
The process of the present invention employs capacitor discharge welding to manufacture powder metal idler sprocket assemblies having complex geometric configurations and integrated features, including multi-tiered and phased powder metal sprocket assemblies. The advantages of using capacitor discharge welding to join powder metal sprocket materials include, for example, low cost formation of required projections in the powder metal tooling; quick formation of the weld so that residual gases, contaminants and lubricant, typically associated with powder metal part porosity, are absent from the weld; and significantly increased robustness of the high carbon weld, due to the plastic phase joint formation coupled with the second temper pulse.